Methods of the above-mentioned type are known from the prior art. Thus, for example, German unexamined patent application DE 24 55 302 A1 discloses a method for analog-to-digital conversion in which an analog input signal undergoes superimposed, equidistant, at least pseudo-stochastically varied sampling intervals at a limited sampling rate. The aim is to achieve increased resolution of the analog input signal without an appreciable increase in the capital costs. It is assumed to be well known that an analog input signal is sampled taking the Shannon or Nyquist theorems into account. Accordingly, the number of samplings per measuring cycle must be greater than 2, and the number of samplings per measuring cycle is an integer, for example as summarized in German unexamined patent application DE 10 2007 043 927 A1. In the cited document, it is also proposed to determine an estimated frequency for the signal frequency, and to determine the sampling frequency as a function of the estimated frequency, and thus, ultimately from the reduced signal frequency. In addition, a method for analog-to-digital conversion is known from unexamined U.S. Pat. No. 5,815,101 A, in which the input signal is sampled with a first sampling rate and with a second sampling rate.
In practice, it has been shown that in the traction network of a motor vehicle, for example, the electrical network is superimposed with interference signals, in particular in the area of a traction battery. During monitoring of the battery cells, the battery voltage or the cell voltage is detected as the input signal. The voltage signal is frequently superimposed with two interference signals having different frequencies, so that reference is made to a high-frequency interference signal and a low-frequency interference signal.
If the signal is now sampled with equidistant sampling points in the measuring cycle, a relatively high measuring error results. The methods proposed in the above-mentioned prior art cannot reduce the measuring error to a satisfactory extent.